Electrolyte composition for screen printing and miniaturized oxygen electrode and production process thereof

ABSTRACT

An electrolyte composition for screen printing, comprising: an organic solvent; an inorganic salt in the form of a fine powder able to pass through a screen printing mesh, the salt powder being dispersed in the organic solvent; and polyvinyl pyrrolidone dissolved in the organic solvent. A miniaturized oxygen electrode having an oxygen sensing site filled with the electrolyte composition. A process for producing a miniaturized oxygen electrode, including a step of patterning or selectively removing an oxygen gas-permeable membrane at a pad region by removing or peeling off an underlying cover film formed thereunder.

This application is a division of application Ser. No. 07/850,834, filedMar. 13,1992, now U.S. Pat. No. 5,281,323.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diaphragm-type miniaturized oxygenelectrode, more particularly, to a miniaturized oxygen electrode usefulfor many applications including a measurement of the dissolved oxygenconcentration of a solution, to an electrolyte composition suitable forforming the sensing site of the miniaturized oxygen electrode, and to aprocess of mass-producing miniaturized oxygen electrodes having auniform quality.

An oxygen electrode is very useful for measuring the dissolved oxygenconcentration in many fields. For example, oxygen electrodes are used inthe field of water control, the BOD (Biochemical Oxygen Demand) in wateris measured, and in the fermentation and brewing field, the dissolvedoxygen concentration of a fermentation tank or fermenter is measured, toensure an efficient fermentation of alcohol, etc.

An oxygen electrode can be combined with an enzyme to form a biosensoror an enzyme electrode to be used for measuring the concentration ofsugar, vitamins, etc. For example, an oxygen electrode can be combinedwith glucose oxidase to measure the concentration of glucose or grapesugar. This utilizes a phenomenon in which glucose is oxidized by thedissolved oxygen with the aid of a catalytic action of glucose oxidaseto form gluconolactone, with a resulting reduction of the dissolvedoxygen amount diffusing into an oxygen electrode.

In addition to the measurement of the dissolved oxygen concentration ofa solution, an oxygen electrode can be advantageously used forcontrolling the oxygen concentration of a gas phase. For example, areduction of the ambient oxygen concentration to below 18% causes adangerous oxygen deficiency, and in medical-care equipment, such asoxygen inhalation and gas anesthetization, the oxygen concentration of agas used must be strictly controlled.

The oxygen electrode is thus very advantageously used in many fields,including environmental instrumentation, the fermentation industry,clinical care, and industrial hygiene.

2. Description of the Related Art

The conventional oxygen electrode typically has a structure as shown inFIG. 1, wherein a vessel or container 118 made of glass, plastics,stainless steel, or the like has an open end (lower end) covered andsealed with an oxygen gas-permeable membrane 107 made of silicone resin,fluororesin or the like, and an aqueous solution 119 of potassiumchloride (KC1), sodium hydroxide (NaOH), etc., is filled in the vessel118, in which an anode 104 made of silver (Ag), lead (Pb), etc., and acathode 105 made of platinum (Pt), gold (Au), etc., are arranged.

The conventional oxygen electrode has a complicated structure, andtherefore, it is difficult not only to miniaturize but also tomass-produce same.

The present inventors and others have proposed a new type ofminiaturized oxygen electrode that can be produced by utilizing asemiconductor production process including a photolithography and ananisotropic etching, as disclosed in Japanese Unexamined PatentPublication (Kokai) No. 63-238,548 and U.S. Pat. No. 4,975,175.

The proposed oxygen electrode has a structure as shown in FIGS. 2 and 3,in which FIG. 2(b) shows an unfinished structure in which an oxygengas-permeable membrane is not yet formed. This structure is produced bythe following sequence. Two grooves 202 to be filled with anelectrolyte-containing material are formed on a silicon wafer 201 by ananisotropic etching and the wafer surface is then covered by an SiO₂insulating layer 203 to form an electrically insulating substrate. Then,two component electrodes, i.e., an anode 204 and a cathode 205, areformed on the insulating layer 203. The anode 204 has one end 204A forexternal electrical connection and the other end of two branchesextending into the grooves 202. The cathode 205 has one end 205A forexternal electrical connection and the other end extending to the topsurface of a plateau retained between the grooves 202. Anelectrolyte-containing material 206 is filled in the grooves 202, andthe filled electrolyte-containing material 206 is in contact with theanode 204 within the grooves 202 and with the cathode 205 on theplateau. The upper surface of the filled electrolyte-containing material206 is then covered with an oxygen gas-permeable membrane 207.

Nevertheless, the step of filling the grooves 202 with theelectrolyte-containing material 206 and the step of covering the filledelectrolyte-containing material 206 with the oxygen gas-permeablemembrane 207 are difficult to carry out in a semiconductor process, andtherefore, are manually carried out chip by chip after the wafer 201 onwhich miniaturized oxygen electrodes have been formed is cut into chipsforming respective oxygen electrodes. The manual operation is a seriousobstacle to the realizing of a mass-production, and further, involvestoo much fluctuation in operation to obtain miniaturized oxygenelectrodes having a stable or uniform performance.

Therefore, it has been desired to provide a structure of a miniaturizedoxygen electrode and a production process thereof in which the fillingof an electrolyte-containing material and the forming of an oxygengas-permeable membrane can be carried out collectively or generally anduniformly, on a wafer as a whole, before the wafer is cut into chips.

The step of filling an electrolyte-containing material has the followingproblems.

The present inventors studied gels containing an aqueous solution ofpotassium chloride and polyelectrolytes and found that, because many ofthese are not photosensitive, the photolithography used in thesemiconductor process cannot be actually applied to the filling of anelectrolyte-containing material.

The electrolyte-containing material must be a liquid having a fluiditywhen it is filled in a groove, and the filled material must form a densefilm after being dried. Also, whether or not the filled materialcontains water significantly affects the quality of an oxygengas-permeable membrane applied on the filled material, and therefore,upon application for an oxygen gas-permeable membrane, theelectrolyte-containing material is preferably dried. The water requiredfor the measurement of the oxygen concentration is supplied as a watervapor through the gas-permeable membrane just before the measurementstarts. The electrolyte-containing material need not contain waterduring the production of an oxygen electrode.

Screen printing is a preferred method of filling anelectrolyte-containing material collectively in a number of miniaturizedoxygen electrodes on a wafer. This screen printing generally uses anemulsion mask and a metal mask to define a printed pattern. An emulsionmask is prepared by applying a photosensitive resin in the form of anemulsion on a mesh of a stainless steel, etc. to provide a printingpattern. Some resins have a transparency which advantageouslyfacilitates the fine alignment required when producing a miniaturizedoxygen electrode because a wafer covered by a resin mask is visiblethrough the resin. The emulsion mask, however, is very weak againstwater, as can be understood from the fact that the developing treatmentof an emulsion is carried out by using water, and the printing of awater-containing substance is difficult. On the other hand, the metalmask is prepared by forming holes in a plate of a stainless steel, etc.,and therefore, is strong against water. The metal mask, however, isdisadvantageous for the fine alignment, because it does not have atransparency. Moreover, the metal mask occasionally provides a printingquality lower than that obtained by the emulsion mask, when using somekinds of printing inks.

The present inventors proposed a process in which anelectrolyte-containing gel is applied by screen printing, i.e. calciumalginate gel, polyacrylamide gel, and agarose gel are printed, asdisclosed in Japanese Unexamined Patent Publication (Kokai) No.1-56,902. This process uses a metal mask to print an aqueous gel andcannot be advantageously used in the production of a miniaturized oxygenelectrode, for the reasons mentioned above. Moreover, a strong filmcannot be obtained because an oxygen gas-permeable membrane is formed ona wet gel.

Potassium chloride is generally used as the electrolyte of an oxygenelectrode. Although potassium chloride is a superior electrolyte, it isnot suitable for use in a miniaturized oxygen electrode because it has adrawback in that it is only soluble in water and that a filled aqueoussolution becomes a white brittle powder when dried. The presentinventors also proposed a polyelectrolyte, as disclosed in JapaneseUnexamined Patent Publication (Kokai) No. 2-240,556. Although this has agood film forming property, the proposed polyelectrolyte is also solubleonly in water, and is difficult to treat because it has a highpolymerization degree and exhibits a high viscosity even as a dilutesolution.

The step of forming an oxygen gas-permeable membrane has the followingproblems.

The gas-permeable membrane is made of silicone resin, fluororesin, orother electrically insulating material. The gas-permeable membrane istherefore formed not to cover the whole surface of a wafer but to have apattern such that the component electrode ends or "pads" 204A and 205for external electrical connection are exposed. The gas-permeablemembrane is formed selectively in the predetermined wafer region otherthan the pad region to be exposed either by applying a resin only to thepredetermined region or by first forming the gas-permeable membrane onthe whole surface of a wafer and then removing the gas-permeablemembrane in the pad region to be exposed.

A screen printing of a liquid resin is known as the former method, i.e.,the selective application of a resin. This method has an advantage inthat a single printing operation simultaneously effects both theapplication and the patterning of a resin, but the silicone resin usedfor forming a gas-permeable membrane is progressively cured by the waterin the ambient air, and therefore, the viscosity of the resin variesduring printing to cause a nonuniform printing, and in the worst case, aclogging of a printing stencil.

A lift-off process using a photoresist is known as the latter method,i.e., the formation and selective removal of a gas-permeable membrane.This process has an advantage in that the semiconductor process isadvantageously applied and a complicated pattern can be easily obtained.This method, however, when applied in the production of a miniaturizedoxygen electrode, provides a completely cured gas-permeable membranehaving a high strength such that the membrane is difficult to peel orexfoliate selectively at the portion to be exposed, even by using anultrasonic treatment. Thus, the lift-off process cannot be practicallyused in the production of a miniaturized oxygen electrode.

U.S. Pat. No. 4,062,750 to J. F. Butler discloses a thin film typeelectrochemical electrode formed on a silicon substrate, having afeature in that an electroconductive layer extends through the siliconsubstrate thickness so that a signal from a sensor disposed on one sideof the substrate is taken out from the other side of the substrate. Asthis electrode does not have the pad portion of the present inventiveelectrode, a gas-permeable membrane may cover the whole surface and apatterning of the membrane for exposing the pad portion is not required.This electrode, however, requires a complicated production process,causing a problem in the practical application. The filling of anelectrolyte is carried out by vacuum deposition, and although sodiumchloride and potassium chloride can be vacuum deposited, many of theinorganic salts used as a buffering agent are deteriorated bydehydration and condensation when exposed to the heat associated withvacuum deposition. Therefore, even when a buffered electrolyte isobtained, the resulting pH will significantly deviate from an expectedvalue and the obtained electrolyte composition must be very restricted,and thus this is not an optimum process. Moreover, problem arises when asingle vacuum deposition apparatus is used for both depositingelectrolytes and for depositing electrode metals, and therefore,individual deposition apparatuses must be provided for the respectivedepositions.

M. J. Madou et al. proposed a microelectrochemical sensor, as disclosedin U.S. Pat. No. 4,874,500 and in AIChE SYMPOSIUM SERIES, No. 267, vol.85, pp. 7-13 (1989). This sensor also has a feature in that anelectroconductive layer extends through the silicon substrate thicknessand a signal from a sensor disposed on one side of the substrate istaken out from the other side of the substrate, and therefore, has thesame drawback as that of J. F. Butler. An electrolyte is filled in sucha manner that an alcoholic solution of poly(hydroxyethylmathacrylate),etc. is painted on, the solvent is evaporated, an electrolyte solutionis introduced to form a gel, and then dried. The conventional problem isapparently eliminated, because an electrolyte is introduced after apolymer is applied, but a crystal grows when a potassium chloridesolution is evaporated. When the amount of potassium chloride is small,the grown crystal is enclosed with the polymer, but when the amount islarge, a number of large crystals appear, which may not be supported bythe polymer. 0n the other hand, the amount of an electrolyte must be aslarge as possible, because the service life of an oxygen electrode isaffected by the electrolyte amount contained therein. Thus, therestricted amount of electrolyte reduces the service life of an oxygenelectrode.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a miniaturized oxygenelectrode which can be mass-produced at a high efficiency bycollectively and uniformly processing a substrate as a whole, aproduction process thereof, and an electrolyte composition able to beadvantageously used therefor.

To achieve the above object according to the first aspect of the presentinvention, there is provided an electrolyte composition for screenprinting, comprising:

an organic solvent;

an inorganic salt in the form of a fine powder able to pass through ascreen printing mesh, the salt powder being dispersed in the organicsolvent; and

polyvinyl pyrrolidone dissolved in the organic solvent.

The electrolyte composition is screen-printed to form anelectrolyte-containing material on a substrate.

The inorganic salt is preferably selected from potassium chloride andsodium chloride.

The inorganic salt used as an electrolyte must be in the form of a finepowder which can pass through the screen printing mesh, for example, inthe form of a fine particle having a diameter not larger than 50 μm.

The organic solvent used in the present invention is preferably analcohol such as butanol, pentanol, or hexanol.

The present inventive electrolyte composition is prepared by dispersingan inorganic salt such as potassium chloride, which is a superiorelectrolyte, in the form of a fine particle adapted for screen printing,in a high molecule polymer dissolved in an organic solvent. The presentinvention uses polyvinyl pyrrolidone as the high molecule polymer. Aninorganic salt such as potassium chloride in the form of a fine particlemay be prepared either by pulverizing a solid material or by pouring anaqueous solution containing an inorganic salt in saturation or in a highconcentration near saturation into an organic solvent such as alcoholand acetone, which can be mixed with water in any proportion, toprecipitate fine particles. Either method provides a powder of fineparticles having a uniform size.

The present inventive electrolyte composition may further comprise abuffering agent, to ensure a constant pH (hydrogen ion concentration) ofthe electrolyte. The buffering agent is a salt exhibiting a bufferingeffect, such as phosphate, acetate, borate, citrate, phthalate,tetraborate, glycine salt, and tris(hydroxymethyl)aminomethane salt, andis used in the form of a fine powder like the potassium chloride powder.

According to the second aspect of the present invention, there is alsoprovided a miniaturized oxygen electrode comprising:

an electrically insulating substrate;

an electrolyte-containing material disposed on the substrate;

a set of component electrodes in contact with the electrolyte-containingmaterial and disposed on the substrate; and

an oxygen gas-permeable membrane covering the electrolyte-containingmaterial;

the electrolyte-containing material being formed by screen-printing onthe substrate the electrolyte composition according to the first aspectof the present invention.

According to the third aspect of the present invention, there isprovided a process of producing a miniaturized oxygen electrode,comprising the steps of:

preparing an electrically insulating substrate;

forming an electrolyte-containing material on the substrate; and

forming on the substrate a set of component electrodes in contact withthe electrolyte-containing material;

forming an oxygen gas-permeable membrane covering theelectrolyte-containing material;

the forming of the electrolyte-containing material being carried out byscreen-printing on the substrate the electrolyte composition accordingto the first aspect of the present invention.

According to the second and third aspects of the present invention, afine powder of an inorganic salt, polyvinyl pyrrolidone, and an organicsolvent are blended to form an electrolyte composition in the form of apaste, which is then applied to a substrate at predetermined portionscollectively by screen printing. The printed electrolyte composition,when dried, forms a dense film such that an oxygen gas-permeablemembrane can be properly formed thereon.

According to the fourth aspect of the present invention, there isprovided a miniaturized oxygen electrode comprising:

an electrically insulating substrate;

an electrolyte-containing material disposed on the substrate;

a set of component electrodes disposed on the substrate, each having anend in contact with the electrolyte-containing material and an end forexternal electrical connection; and

an oxygen gas-permeable membrane covering the substrate in a portioncontaining the electrolyte-containing material;

the oxygen gas-permeable membrane being removed from the substrate in aregion containing the end for external electrical connection, byremoving a removable cover film interposed between the substrate and theoxygen gas-permeable membrane.

According to the fifth aspect of the present invention, there isprovided a process for producing a miniaturized oxygen electrode,comprising the steps of:

preparing an electrically insulating substrate;

forming an electrolyte-containing material on the substrate;

forming on the substrate a set of component electrodes each having anend in contact with the electrolyte-containing material and an end forexternal electrical connection; and

forming a removable cover film on the substrate in a region to beexposed in the following removing step, the region containing thecomponent electrode end for external electrical connection;

forming an oxygen gas-permeable membrane covering the substrate surfaceincluding the region of the removable cover film; and

removing the oxygen gas-permeable membrane by peeling the removablecover film away from the substrate surface, to expose the to-be-exposedregion of the substrate and thereby shape the oxygen gas-permeablemembrane to a predetermined pattern.

The process according to the fifth aspect of the present inventionpreferably comprises the steps of:

screen-printing a thermosetting resin onto the to-be-exposed region ofthe substrate;

heating the resin to cure the resin to form a resin film as theremovable cover film;

forming the oxygen gas-permeable membrane covering the substrate surfaceincluding the region of the resin film;

peeling the resin film to expose the to-be-exposed region, and therebyshape the oxygen gas-permeable membrane to a predetermined pattern.

According to the fourth and the fifth aspects of the present invention,an oxygen gas-permeable membrane is formed selectively or patterned tocover the necessary region of the substrate surface by first covering aregion of substrate to be exposed with a removable cover film, applyinga resin for forming an oxygen gas-permeable membrane onto the wholesurface of the substrate by spin coating, and then peeling orexfoliating the removable cover film to thereby remove the oxygengas-permeable membrane together with the cover film in the region ofsubstrate to be exposed. The present invention uses, as the material ofthe removable cover film, a thermosetting resin, a solution ofpolyvinylchloride in an organic solvent, or other resins. Such resinsare applied to the predetermined region of a substrate by screenprinting, and then cured by heating or drying to form a removable coverfilm.

The electrically insulating substrate, on which the present inventiveminiaturized oxygen electrode is formed, may be an electricallyinsulating substrate having a flatness and a smoothness sufficient forforming a miniaturized oxygen electrode by using the semiconductorprocess. A silicon wafer is most advantageously used as the insulatingsubstrate, from the viewpoint of the application of the productionprocess of silicon semiconductors currently most generally used.

The present invention may be directly applied to miniaturized oxygenelectrodes formed on insulating substrates other than the silicon wafer.Namely, a miniaturized oxygen electrode is produced by using a flatsubstrate of an electrically insulating substance such as glass, quartzand plastics, in such a manner that a component electrode pattern isformed on the substrate, an electrolyte-containing material is filled inthe oxygen sensing site by screen printing an electrolyte composition ofthe present invention, and then an oxygen gas-permeable membrane isselectively formed or patterned by the steps including forming aremovable cover film by screen-printing a thermosetting resin, etc., onthe pad portion, i.e., the region of the substrate including thecomponent electrode end for external electrical connection. It will beeasily understood that, even in this case, the present invention alsoprovides an advantage in that a number of miniaturized oxygen electrodesare formed collectively at one time on the whole region of an integralsubstrate.

According to the present invention, the screen-printed electrolytecomposition contains a fine powder of an inorganic salt or anelectrolyte not dissolved but dispersed in an organic solvent, andtherefore, the inorganic salt, even when dried, does not form a brittlecrystal but remains a fine powder, and this enables anelectrolyte-containing material in the form of a dense solid material tobe formed. The thus obtained electrolyte-containing material isessentially composed of the inorganic salt and polyvinyl pyrrolidone.Upon operating a miniaturized oxygen electrode, water is introduced intothe electrolyte-containing material. Both the inorganic salt andpolyvinyl pyrrolidone are water-soluble and completely dissolved in theintroduced water, and thus the present inventive electrolyte-containingmaterial satisfies the requirement for a miniaturized oxygen electrodethat it remains in a solid state during the formation of an oxygengas-permeable membrane and forms an aqueous solution when theminiaturized oxygen electrode is operated.

Potassium chloride and sodium chloride are superior electrolytes and canbe advantageously used as the inorganic salt according to the presentinvention, to obtain the best performance of a miniaturized oxygenelectrode.

The addition of a salt having a pH buffering effect, such as phosphate,to the present inventive electrolyte composition ensures that theelectrolyte has a constant pH. As the electrochemical reaction in theoxygen electrode depends on the pH value, the constant pH improves thestability of the oxygen electrode performance.

The miniaturized oxygen electrode according to the present invention isproduced by filling an electrolyte composition containing an inorganicsalt as an electrolyte in the form of a fine powder, collectively in allof the predetermined portions of a substrate, by screen printing, tothereby ensure a uniform filling operation and a high productivity.

The process of producing a miniaturized oxygen electrode according tothe present invention fills an electrolyte composition containing aninorganic salt as an electrolyte in the form of a fine powder,collectively in all of the predetermined portions of a substrate byscreen printing, and thereby ensures a uniform filling operation andmass-production of a miniaturized oxygen electrode even when the filledportion has a complicated shape. The present inventive productionprocess can also advantageously cope with any increase in the number offilling portions associated with an enlargement of the substrate size.

The miniaturized oxygen electrode according to the present inventionensures a high productivity even when the oxygen gas-permeable membraneand the exposed portion have a complicated shape, because the oxygengas-permeable membrane is patterned (or selectively formed) by removinga cover film formed in a predetermined shape. The oxygen gas-permeablemembrane is applied collectively on the entire substrate surface byspin-coating, and thereby a high productivity is ensured and an oxygengas-permeable membrane having a uniform thickness over the entiresubstrate surface is formed.

The miniaturized oxygen electrode according to the present invention canbe produced at a high productivity by effectively forming an oxygengas-permeable membrane having a uniform thickness over the entiresubstrate surface, i.e., by first covering a substrate region to beexposed with a removable cover film, forming an oxygen gas-permeablemembrane collectively on the entire substrate surface, and then peelingor exfoliating the removable cover film to selectively form or patternthe oxygen gas-permeable membrane.

The process according to the present invention can easily cope with acomplicated pattern of oxygen gas-permeable membrane and with anyincrease in the number of portions to be exposed, because an oxygengas-permeable membrane is patterned through the steps of: applying athermosetting resin of a resin dissolved in an organic solvent to theto-be-exposed portions by screen printing; curing the applied resin byheating or drying to form a removable cover film; and then peeling orexfoliating the removable cover film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the essential arrangement of a conventional oxygenelectrode, in Section view;

FIGS. 2 (a) through (c) show a miniaturized oxygen electrode in planview (a, b) and sectional view (c);

FIGS. 3(a) through (n) show a process sequence according to the presentinvention, in sectional and plan views;

FIG. 4 is a graph showing a typical response of a miniaturized oxygenelectrode according to the present invention, in terms of therelationship between the time elapsed from the addition of Na₂ SO₃ andthe output current;

FIG. 5 is a graph showing the linear calibration curve of a miniaturizedoxygen electrode according to the present invention, in terms of therelationship between the dissolved oxygen content and the outputcurrent;

FIGS. 6(a) through (f) show a process sequence according to the presentinvention, in sectional and plan views;

FIGS. 7(a) through (l) show a process sequence according to the presentinvention, in sectional and plan views;

FIGS. 8(a) through (c) show a three-pole miniaturized oxygen electrode;

FIGS. 9(a) through (e) show a process sequence for producing athree-pole miniaturized oxygen electrode, according to the presentinvention, in sectional and plan views;

FIG. 10 shows a miniaturized oxygen electrode mounted on an adapter foruse in a fermenter, in sectional view; and

FIG. 11 shows an arrangement of a device for measuring the oxygenconcentration in which a miniaturized oxygen electrode according to thepresent invention is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Referring to FIG. 3, a process sequence for producing a miniaturizedoxygen electrode according to the present invention by using a siliconwafer will be described below. Although the sequence is described forthe case in which a miniaturized oxygen electrode is formed on a 2-inchsilicon wafer, for simplicity, essentially the same process sequence canbe also used for a larger wafer. The Figures depict the wafer in whichthe corresponding process step is completed.

Step 1 Cleaning Wafer

A 2-inch silicon wafer 301 (400 μm thick, (100) plane) was thoroughlycleaned with a mixed solution of hydrogen peroxide and ammonia and witha concentrated nitric acid.

Step 2 Forming SiO₂ Layer (FIG. 3(a))

The wafer 301 was wet-thermally oxidized at 1000° C. for 200 min. toform a 0.8 μm thick SiO₂ layer 312 on both sides of the wafer. The SiO₂layer 312 is to be patterned in the following step 4 and used as a maskwhen anisotropically etching the silicon wafer in the following step 5.

Step 3 Forming Resist Pattern (FIG. 3(b))

A negative-type photoresist (Tokyo Ohka Kogyo Co., Ltd., OMR-83,viscosity 60 cP) was applied on the entire upper surface of the wafer,prebaked at 80° C. for 30 min. and was subjected to a photolithographytreatment to form a resist pattern 313. The resist pattern 313 coversthe upper surface of the wafer 301 except for a region 302A at whichgrooves 302 (FIG. 3(d)) for receiving an electrolyte-containing materialare to be formed in the following step 5. The resist pattern 313 servesas a mask upon etching the SiO₂ layer 312 in the following step 4. Thesame photoresist was applied on the lower surface of the wafer 301,which was then baked at 150° C. for 30 min.

Step 4 Etching SiO₂ Layer (FIG. 3(c))

The wafer 301 was immersed in en etchant for SiO₂ (50% HF/1 ml+NH₄ F/6ml) to partially remove the SiO₂ layer 312 in the portion 302A notcovered with the photoresist 313. The wafer 301 was then immersed in amixed solution of connected sulfuric acid and hydrogen peroxide, toremove the photoresist 313.

Step 5 Anisotropically Etching Silicon Wafer FIG. 3(d))

The wafer 301 was immersed in an etchant for silicon (35% KOH) at 80° C.to anisotropically etch the silicon wafer 301 by using the SiO₂ layer asa mask, and thereby forming two 300 μm deep grooves 302 for receiving anelectrolyte-containing material. After the anisotropic etching wasfinished, the wafer 301 was cleaned with pure water.

Step 6 Removing SiO₂ Layer (FIG. 3(e))

Subsequent to the water cleaning, the SiO₂ layer 312 was removed by thesame operation as that performed in Step 4.

Step 7 Forming SiO₂ Layer (FIG. 3(f))

The same operations as performed in Steps 1 and 2 were carried out toeffect a thermal oxidation of the wafer 301, and thereby form a 0.8 μmthick SiO₂ layer 303 on the entirety of both sides of the wafer 301. Thethus-formed SiO₂ layer 303 functions as an insulating layer of aminiaturized oxygen electrode or the final product.

Step 8 Forming Thin Layers of Chromium and Silver (FIGS. 3(g1), 3(g2))

A 400 Å thick chromium thin layer 314 and a 4000 Å thick silver thinlayer 315 overlying on the chromium layer 314, were formed on the entireupper surface of the wafer 301 by vacuum deposition. The silver thinlayer 315 is an electroconductive layer composing the substantialportion of component electrodes (anode and cathode) and the chromiumthin layer 314 is a ground layer for ensuring an adhesion of the silverthin layer 315 to the SiO₂ insulating layer 303 formed on the wafer 301.

Step 9 Forming Photoresist Pattern (FIGS. 3(h1), 3(h2))

This step provides a photoresist pattern 316 to be used as a mask in thefollowing Steps 10 and 11, in which the silver thin layer 315 and thechromium thin layer 314 are etched to thereby effect a patterning ofcomponent electrodes (anode and cathode) of a miniaturized oxygenelectrode.

A positive-type photoresist (Tokyo Ohka Kogyo Co., Ltd., OFPR-800,viscosity 20 cP or OFPR-5000, viscosity 50 cP) was dropped on the wafer301 to uniformly cover the wafer 301. The photoresist is preferablydropped in an amount such that it spreads just to the wafercircumferential edge. The wafer 301 was prebaked at 80° C. for 30 min.

The wafer 301 was pattern-aligned with a glass mask by a mask aligner,exposed to light, and developed to form a photoresist pattern 316. Theexposure and development cycle was repeated to ensure a completeexposure of the positive-type photoresist layer, which is too thick tocomplete the exposure over the thickness at one time.

Step 10 Etching Thin Layers of Silver and Chromium (FIGS. 3(i1) and3(i2))

The wafer 301 was immersed in an etchant for silver (NH₃ water/1 ml+H₂O₂ /1 ml+water/20 ml) to remove a bare portion of the silver layer, andthereby form the substantial portion of component electrodes.

The wafer was then immersed in an etchant for chromium (NaOH/0.5 g+K₃Fe(CN)₆ /1 g+water/4 ml) to remove a bare portion of the chromium layer314.

Step 11 Forming Photoresist Pattern (FIG. 3(j1) and 3(j2)

This step provides a photoresist pattern 317 for defining the oxygensensing site of a miniaturized oxygen electrode.

A layer 317 of a negative-type photoresist (Tokyo Ohka Kogyo Co., Ltd.,OMR-83, viscosity 60 cP) was formed on the wafer 301 to cover the wafersurface in the portion other than a region 309 of the oxygen sensingsite (two grooves and a flat plateau therebetween) and a pad region 311,at which the pad portions 304A and 305A of component electrodes 304 and305 are to be formed. This is performed by applying the photoresist tothe wafer surface, prebaking the wafer at 80° C. for 30 min, andexposing to light and developing the photoresist layer. Thereafter, thephotoresist layer was postbaked at 150° C. for 30 min.

Step 12 Screen-printing Electrolyte Composition (FIGS. 3(k1), 3(k2) and3(k3))

An electrolyte composition was screen-printed at the oxygen sensing site309 (two grooves and a flat plateau therebetween) defined by thephotoresist 317, to form an electrolyte-containing material 306. Thepreparation of the electrolyte used will be described later.

Step 13 Forming Pad Region Cover Film (FIGS. 3(l1) and 3(l2))

A thermosetting release coating (Fujikura Kasei Co. XB-801) wasscreen-printed at the pad region 311 at a thickness of 100 μm and curedby heating at 150° C. for 10 min. to form a removable cover film 308.

Step 14 Forming Oxygen Gas-Permeable Membrane (FIGS. 3(m1) and 3(m2))

An oxygen gas-permeable membrane 307 having a double-layered structurewas formed on the wafer 301 to entirely cover the upper surface of thewafer 301. The lower layer of the membrane 307 was first formed byapplying a negative-type photoresist (Tokyo Ohka Kogyo Co., Ltd.,OMR-83, viscosity 100 cP) to the wafer 301 by spin coating, prebaking at80° C. for 30 min., exposing the entire wafer surface to light anddeveloping, and postbaking at 150° C. for 30 min. The upper layer of themembrane 307 was then formed by applying a silicone resin (Toray-DowCorning Silicone Co. SE9176) to the wafer 301 by spin coating and curingthe coated resin by heating at 70° C. for 30 min. in an oven moistenedby water contained in a Petri dish or a beaker placed in the oven.

Step 15 Exposing Pads (FIGS. 3(n1) and 3(n2))

The cover film 308 formed on the pad region 311 was peeled with apincette to selectively remove the oxygen gas-permeable membrane in thatregion, and thereby expose the pads 304A and 305A of a miniaturizedoxygen electrode.

Step 16 Separating Miniaturized Oxygen Electrodes

A number of miniaturized oxygen electrodes were collectively formed onthe wafer 301 at one time by the preceding Steps 1 through 15 and werecut into chips by a dicing saw. The shown example provides forty chipsof miniaturized oxygen electrodes at one time.

EXAMPLE 2

Miniaturized oxygen electrodes were produced by the same processsequence as that of Example 1, except that Step 13 of forming a padregion cover film was modified as follows:

Step 13 Forming Pad Region Cover Film (Modified)

Polyvinylchloride resin dissolved in tetrahydrofuran was screen-printedat the pad region 311 at a thickness of 50 μm and cured by heating at70° C. to form a cover film 308.

The electrolyte composition according to the present invention used inStep 12 of Examples 1 and 2 was prepared in the following manner.

Preparation Procedure 1: Providing Fine Powder of Inorganic salt

Fine particles of potassium chloride or sodium chloride were formed byeither of the following procedures (a) and (b):

(a) A solid material of potassium chloride or sodium chloride waspulverized to fine particles having a diameter of 10 μm or less by apulverizer (Fritsch Co. Type P-5).

(b) A saturated aqueous solution of potassium chloride or sodiumchloride was prepared. The solution was poured into an organic solventsuch as ethanol, propanol, or acetone of an amount of ten times thesolution, through a Teflon* ball filter (Iuchiseieido Co., pore diameter10 μm). The organic solvent was thoroughly agitated by a stirrer duringthe pouring. This provided a precipitation of fine particles ofinorganic salt, which was collected by a glas filter, washed two orthree times with a fresh organic solvent of the same kind, and dried toobtain fine particles having a diameter of 10 μm or less.

Preparation Procedure 2: Blending Electrolyte. Composition

The above-obtained fine particles of inorganic salt, polyvinylpyrrolidone, and an organic solvent were blended to form an electrolytecomposition in the form of a paste. The following is an example of thethus-blended composition.

Electrolyte Composition: Case 1

    ______________________________________                                        Potassium chloride fine particle                                                                   0.25 g                                                   Polyvinyl pyrrolidone                                                                              1 g                                                      Pentanol             5 g                                                      ______________________________________                                    

The blending may be carried out in a manner such that the electrolytecomposition contains 30 to 70% of a solid part and the remainder of anorganic solvent, the solid part containing 50 to 90% of an inorganicsalt. The following is an example of the thus-blended composition.

Electrolyte Composition: Case 2

    ______________________________________                                        Potassium chloride fine particle                                                                    4 g                                                     Polyvinyl pyrrolidone 1 g                                                     Pentanol              5 g                                                     ______________________________________                                    

According to preferred embodiment of the present invention, anelectrolyte composition further comprises a salt having a pH-bufferingeffect. Although a phosphate was added in the following case, thebuffering agent used in the present invention may be selected from thegroup consisting of phosphates, accetates, borates, citrates,phthalates, tetraborates, glycine salts, andtris(hydroxymethyl)aminomethane salts.

An electrolyte composition with an addition of a phosphate as abuffering agent may be prepared in the following manner, for example.

Preparation Procedure 1:

Providing Fine Powder of Inorganic Salt

Fine particles of potassium chloride or sodium chloride were formed byeither of the following procedures (a) and (b):

(a) 74.55 g of potassium chloride and 8.71 g of dipotassium hydrogenphosphate were weighed and pulverized to particles having a diameter of10 μm or less by a pulverizer (Fritsch Co., Type P-5).

(b) 74.55 g of potassium chloride and 8.71 g of dipotassium hydrogenphosphate were weighed and dissolved in 230 ml of water. The aqueoussolution was poured into an amount of ethanol ten times the amount ofthe solution, through a Teflon ball filter (Iuchiseieido Co., porediameter 10 μm). The ethanol was thoroughly agitated by a stirrer duringthe pouring. This resulted in a precipitation of fine particles ofinorganic salt, which was then collected by a glass filter, washed witha fresh ethanol two or three times, and dried to obtain fine particleshaving a diameter of 10 μm or less.

The fine particles of potassium chloride or sodium chloride and the fineparticles of phosphate or a buffering agent may be separately prepared.For example, when a concentrated aqueous solution of potassium chlorideor sodium chloride is formed, an aqueous solution of potassiumdihydrogen phosphate and sodium dihydrogen phosphate (4:6 in molarratio) can be separately formed. Both solutions are preferably in asaturation state, which provides a greater amount of fine particles,i.e., a high efficiency. Note that the weighed phosphates must becompletely dissolved in water, because the proportion of the dissolvedphosphates significantly affects the pH value. The thus-prepared aqueoussolutions are poured into an organic solvent such as ethanol, in thesame manner as described above, respectively, and the precipitated fineparticles are collected.

Preparation Procedure 2: Blending Electrolyte Composition

The above-obtained fine particles of inorganic salts, polyvinylpyrrolidone, and an organic solvent were blended to form an electrolytecomposition in the form of a paste. The followings are examples of thethus-blended compositions.

Electrolyte Composition: Case 3

    ______________________________________                                        Mixture of fine particles of                                                                        0.25 g                                                  potassium chloride and phosphate                                              Polyvinyl pyrrolidone 1 g                                                     Pentanol              5 g                                                     ______________________________________                                    

Electrolyte Composition: Case 4 (fine particles of buffering agentseparately formed)

    ______________________________________                                        Potassium chloride fine particle                                                                   3.5 g                                                    Phosphate fine particle                                                                            0.5 g                                                    Polyvinyl pyrrolidone                                                                              1 g                                                      Pentanol             5 g                                                      ______________________________________                                    

The performance of the miniaturized oxygen electrode produced inExamples 1 and 2 was tested by measuring the dissolved oxygenconcentration of a 10 mM buffered phosphoric acid solution having a pHvalue of 7.0 at an applied voltage of 0.6 V and a temperature of 25° C.

FIG. 4 shows a response curve observed when sodium sulfite is added to asolution saturated with 100% oxygen, to instantaneously reduce theoxygen concentration to zero. The response time was 40 seconds, whichcorresponded to the variation of the dissolved oxygen concentration.

FIG. 5 shows a calibration curve obtained in this case, from which it isseen that a good linearity is ensured over the entire range of thedissolved oxygen concentration of from 0 ppm through 8 ppm, i.e., thesaturation concentration.

EXAMPLE 3

Referring to FIG. 6, a process sequence for producing a miniaturizedoxygen electrode according to the present invention by using anelectrically insulating flat substrate other than a silicon wafer willbe described.

Step 1 Forming Component Electrode Pattern (FIG. 6(a))

A 60 mm square, 1.6 mm thick, cleaned electrically insulating flatsubstrate 401 was prepared. The insulating substrate 401 may be made ofglass, quartz, ceramics, plastics or other electrically insulatingsubstances.

A component electrode pattern consisting of an anode 404 and a cathode405 was formed on the insulating substrate 401 by either of thefollowing procedures (a) and (b):

(a) A silver thin layer is formed by vacuum deposition and is etched toform a predetermined electrode pattern, in the same manner as used inpreceding Examples 1 and 2.

(b) An electroconductive paste (Fujikura Kasei Co., D-1230 modified) isscreen-printed on the substrate.

The component electrodes 404 and 405 have ends for external electricalconnections or pads 404A and 405A, respectively.

An auxiliary pad 420 provided between the pads 404A and 405A can be usedfor a miniaturized oxygen electrode having a three-pole structure, forexample.

Step 2 Screen-Printing Electrolyte Composition (FIG. 6(b))

The same electrolyte composition as used in Example 1 was screen-printedto fill a region 409 of the oxygen sensing site, and thereby form anelectrolyte-containing material 406.

Step 3 Forming Pad Region Cover Film (FIG. 6(c))

A thermosetting release coating (Fujikura Kasei Co., XB-801) wasscreen-printed at a pad region 411 containing the pads 404A and 405A andthe auxiliary pad 420, to form a cover film 408 covering the pad region411.

Step 4 Forming Oxygen Gas-Permeable Membrane (FIG. 6(d))

An oxygen gas-permeable membrane 407 having a double-layered structurewas formed on the substrate 401 to entirely cover the upper surface ofthe substrate 401. The lower and the upper layers of the membrane 407were formed by applying a negative-type photoresist (Tokyo Ohka KogyoCo., Ltd., OMR-83, viscosity 100 cP) and a silicone resin (Toray-DowCorning Silicone Co., SE9176) by spin coating, respectively, and thencuring the applied layers.

Step 5 Exposing Pad Region (FIG. 6(e))

The cover film 408 formed on the pad region 411 was peeled with apincette to selectively remove the oxygen gas-permeable membrane 407 inthat portion, and thereby expose the pads 404A and 405A of aminiaturized oxygen electrode. The auxiliary pad 420 was simultaneouslyexposed.

Step 6 Separating Miniaturized Oxygen Electrodes (FIG. 6(f))

A plurality of miniaturized oxygen electrodes were collectively formedon the electrically insulating substrate 401 at one time by thepreceding Steps 1 to 6, and were cut into chips by a dicing saw. Theshown example provided seven chips of miniaturized oxygen electrodes 418from a single substrate, simultaneously.

Although the preceding Examples formed the component electrodes ofsilver, the component electrodes may be formed of gold instead ofsilver, or a cathode and an anode may be formed of gold and silver,respectively.

For example, the component electrodes can be formed of gold instead ofsilver by a partial modification of the process steps of Example 1, asfollows.

EXAMPLE 4

Steps 8 and 10 of Example 1 were modified in the following manner.

In Step 8 (FIGS. 3(g1) and 3(g2)), the same operation was performed asin Example 1, except that a gold thin layer 315 (4000 Å thick) wasvacuum deposited instead of the silver thin layer 315 (4000 Å thick).

The subsequent Step 9(FIGS. 3(h1) and 3(h2)) was performed in the samemanner as in Example 1.

In Step 10 (FIGS. 3(i1) and 3(i2)), the same operation was performed asthat in Example 1, except that the wafer 301 was immersed in an etchantfor gold (KI/4 g+I₂ /1 g+water/40 ml) instead of the etchant for silver.

These modifications provided a miniaturized oxygen electrode having acomponent electrode formed of gold.

A miniaturized oxygen electrode having a gold cathode and a silver anodemay be produced in the following manner.

EXAMPLE 5

Referring to FIG. 7, a process sequence for producing a miniaturizedoxygen electrode having a gold cathode and a silver anode according tothe present invention by using a glass substrate will be described.

Step 1 Cleaning substrate (FIG. 7(a))

A 60 mm square, 1.6 mm thick glass substrate 511 was thoroughly washedwith a detergent (for example, Furuuchi Kagaku Co., Semico Clean 56) andacetone.

Step 2 Forming Thin Layers of Chromium, Gold and Silver (FIG. 7(b))

A chromium thin layer (400 Å thick, for example), a gold thin layer(4000 Å, for example) and a silver thin layer 512 (4000 Å thick, forexample) were formed on the substrate 511, in that order, by a vacuumdeposition. The chlomium thin layer ensures a good adhesion between theglass substrate 511 and component electrodes of gold and silver.

Step 3 Forming Photoresist Pattern (FIG. 7(c))

A positive-type photoresist (for example, Tokyo Ohka Kogyo Co., Ltd.,OFPR-800, 20 cP or OFPR-5000, 50 cP) was applied on the silver thinlayer 512 and prebaked at 80° C. for 30 min. The thus-formed photoresistlayer was exposed to light and developed to form a photoresist pattern513 corresponding to all component electrodes.

Step 4 Etching Gold and Silver Thin Layers (FIG. 7(d))

The substrate 511 was immersed in an etchant for silver (for example,29%NH₄ OH/1 ml+31%H₂ O₂ /1 ml+water/20 ml) to pattern the silver thinlayer 512. The substrate 511 was then immersed in an etchant for gold(for example, KI/4 g+I₂ /1 g+water/40 ml) to pattern the gold thinlayer.

This exposed the chromium thin layer 514 in the portion not covered withthe photoresist layer.

Step 5 Re-Patterning Photoresist Pattern (FIG. 7(e))

The positive-type photoresist layer 513 was exposed to light anddeveloped again so that the photoresist pattern 513 remained only in theportion at which an anode is to be formed, and the other portion of thephotoresist pattern 513 was removed to expose the silver thin layer 512.

Step 6 Patterning Component Electrodes (FIG. 7(f))

The substrate 511 was immersed in an etchant for silver to remove thesilver thin layer exposed in the preceding Step 5, and thereby exposethe underlying gold thin layer, with the result that the gold cathode504, including part of the extended card edge portion (or pad) 503, andpart of a floating card edge portion (or pad), were exposed. Thesubstrate was then immersed in an etchant for chromium (for example,NaOH/0.5 g+K₃ Fe(CN)₆ /1 g+water/4 ml) to remove an open portion of thechromium thin layer 514. The substrate was immersed in acetone toentirely remove the photoresist pattern 513, and thereby expose thesilver anode 505 including part of the extended card edge portion (orpad) 503.

This completed the formation of the entire arrangement of componentelectrodes including the gold cathode 504 and the silver anode 505.

Step 7 Forming Photoresist Pattern. (FIG. 7(g))

A negative-type photoresist (for example, Tokyo Ohka Kogyo Co., Ltd.,OMR-83, 60 cP) was applied to the entire upper surface of the substrate511 by spin coating and prebaked at 70°-180° C. for 30 min. After anexposure to light and development, the photoresist was postbaked at 150°C. for 30 min. to form a photoresist pattern 516, which covered thesubstrate surface except for an oxygen sensing site of the silver anode505, part of the gold cathode 504, and the card edge portion (or pad)503.

Step 8 Screen-Printing Electrolyte Composition (FIG. 7(h))

An electrolyte composition of the present invention was screen-printedon the oxygen sensing site 515 defined by the photoresist pattern 516,to form an electrolyte-containing material 517.

Step 9 Forming Pad Region Cover Film (FIG. 7(i))

A thermosetting release coating (Fujikura Kasei Co., XB-801) wasscreen-printed on the pad region (or card edge portion) 503 at athickness of 100 μm, and then cured by heating at 150° C. for 10 min. toform a cover film 508.

Step 10 Forming Oxygen Gas-Permeable Membrane (FIG. 7(j))

A oxygen gas-permeable membrane 507 having a double-layered structurewas formed on the glass substrate 511 to entirely cover the substrateupper surface. The lower layer of the membrane 507 was first formed byspin-coating a negative-type phtoresist (Tokyo Ohka Kogyo Co., Ltd.,OMR-83, viscosity 100 cP), prebaking at 80° C. for 30 min., exposing theentire substrate surface to light, and postbaking at 150° C. for 30 min.The upper layer was then formed by spin-coating a silicone resin(Toray-Dow Corning Silicone Co., SE9176) and curing by heating at 70° C.for 30 min. in an oven moistened with the water contained in a Petridish or a beaker placed in the oven.

Step 11 Exposing Pads (FIG. 7(k))

The cover film 508 formed in the pad region 503 was peeled off with apincette to selectively remove the oxygen gas-permeable membrane 507 inthat portion, and thereby expose the pads (or card edges) 504A and 505Aof a miniaturized oxygen electrode.

The selective removal of the oxygen gas-permeable membrane 507 waseffected in such a way that, when the cover film 508 was peeled off, theoxygen gas-permeable membrane 507 was cut by the edge of the cover film508 between the membrane portion positioned on the cover film 508 andthe other membrane portion away from the cover film 508.

The portion of oxygen gas-permeable membrane remaining on the glasssubstrate strongly adhered to the substrate and was not exfoliated bythe later treatments, including a water vapor treatment describe later.The oxygen gas-permeable membrane also ensures a high reliability suchthat it does not fracture when attached to a catheter and used in amedical care, or when used for monitoring the oxygen concentration in afermenter subjected to a sterilization at a temperature of 120° C. and adifferential pressure of 1.2 atm. for about 15 min.

Step 12 Separating Miniaturized Oxygen Electrodes (FIG. 7(l))

A plurality of miniaturized oxygen electrodes were collectively formedon the glass substrate 511 at one time and were cut into chips by adicing saw. The shown example provides seven miniaturized oxygenelectrodes from a single substrate at one time.

The oxygen gas-permeable membrane strongly adhered to the substrate anddid not exfoliate during a cutting thereof along a scribe line, andfurther, did not exhibit a lowered reliability when subjected to areliability test.

The miniaturized oxygen electrode according to the present invention canbe applied to any clark type device for electrochemically detectingoxygen, including Galvani type, and three-pole type oxygen electrodes.

FIGS. 8(a), (b) and (c) show an example of the three-pole typeminiaturized oxygen electrode, wherein FIG. 8(b) shows an unfinishedstructure in which an oxygen gas-permeable membrane is not yet formed.

A working electrode 702, a counter electrode 703 and a referenceelectrode 704 are formed on a silicon wafer 701 (see FIG. 8(b)) and anoxygen gas-permeable membrane 705 covers the surface except for pads702A, 703A and 704A of the respective electrodes. FIG. 8(c) shows anI--I section of an oxygen sensing site, in which an electrolytecomposition 715 is filled in grooves formed in the silicon wafer to forma electrolyte-containing material.

EXAMPLE 6

A three-pole type miniaturized oxygen electrode according to the presentinvention and having a basic structure as shown in FIGS. 8(a) to (c) wasproduced according to the present invention in the following sequence.

Step 1 Forming Grooves for Receiving Electrolyte-Containing Material(FIG. 9(a1) and 9(a2))

In the same sequence as carried out in Steps 1 through 7 of Example 1,grooves 706 for receiving an electrolyte-containing material and an SiO₂insulating layer 707 were formed on both sides of a silicon wafer 701.

Step 2 Forming Component Electrode Pattern (FIGS. 9(b1) and 9(b2)

In the same sequence as carried out in Steps 2 through 6 of Example 5, aworking electrode 702 and a counter electrode 703, both of gold, and areference electrode 704 of silver were formed.

Step 3 Forming Photoresist Pattern (FIGS. 9(c1) and 9(c2))

By the same operation as carried out in Step 11 of Example 1, aphotoresist pattern 711 was formed to cover the substrate surface exceptfor a region 712 of the oxygen sensing site and a pad region 713.

Step 4 Screen-Printing Electrolyte Composition (FIGS. 9(d1) and 9(d2))

By the same operation carried out in Step 12 of Example 1, anelectrolyte composition 715 was screen-printed on the oxygen sensingsite 712.

Step 5 Forming Pad Region Cover Film (not shown).

By the same operation as carried out in Step 13 of Example 1, aremovable cover film was formed.

Step 6 Forming Oxygen Gas-Permeable Membrane (not shown)

By the same operation as carried out in Step 14 of Example 1, an oxygengas-permeable membrane was formed.

Step 7 Exposing Pads (FIGS. 9(e1) and 9(e2)

By the same operation as carried out in Step 15 of Example 1, pads 702A,703A and 704A were exposed.

Step 8: Separating Miniaturized Oxygen Electrodes (not shown)

By the same operation as carried out in Step 16 of Example 1, a numberof miniaturized oxygen electrode formed on the silicon wafer were cutinto chips.

In Examples 1 through 6, miniaturized oxygen electrodes were produced ata yield of 98% or more and exhibited a good response characteristic,i.e., an output fluctuation of less than ±3% when measured in watersaturated with oxygen.

The produced miniaturized oxygen electrode is preserved in the driedcondition and can be made operative when supplied with water through theoxygen gas-permeable membrane by water vapor sterilization (for example,at 121° C. and 2.2 atm.), immersion in water, exposure to a saturatedwater vapor, etc.

When an miniaturized oxygen electrode is used for a fermenter, theabove-mentioned preparation or water supply may be conveniently effectedtogether with sterilization of the culture medium. As shown in FIG. 10,a miniaturized oxygen electrode 801 of the present invention isconveniently attached to a special adaptor 802 designed for a fermenter(proposed by the present inventors and others in Japanese PatentApplication No. 1-231,708).

The external electrical connection of a miniaturized oxygen electrode isusually carried out by inserting the card edge portion (or pad portion)503 to a card edge connecter (for example, Fujitsu Ltd., Type 760).

FIG. 11 shows an arrangement of an oxygen concentration measuring devicein which a miniaturized oxygen electrode of the present invention isused. An oxygen concentration measuring device 810 is composed of aminiaturized oxygen electrode 819 of the present invention and acontroller 820. The controller 820 is composed of a voltage supply unit821 for generating a voltage to be supplied to the oxygen electrode 819,a current-to-voltage converter unit 822 for converting an output currentfrom the oxygen electrode 819 to a voltage, a calibration unit 823 forcalibrating an output voltage from the converter unit 822 at the oxygenconcentrations of 0% and 100%, and a display unit 824. The device 810measures the dissolved oxygen concentration in many kinds of solutionsand the oxygen concentration of gas phases.

As herein described, the present invention provides a miniaturizedoxygen electrode which can be mass-produced at a high efficiency bycollectively and uniformly processing a substrate as a whole by usingthe semiconductor process, a production process thereof, and anelectrolyte composition able to be advantageously used therefor.

We claim:
 1. An electrolyte composition for screen printing,comprising:an organic solvent; a water soluble inorganic salt in theform of a fine powder of a size to pass through a screen printing mesh,said salt powder being dispersed in said organic solvent, said inorganicsalt being selected from potassium chloride and sodium chloride; andpolyvinyl pyrrolidone dissolved in said organic solvent.
 2. Acomposition according to claim 1, which further comprises a bufferingagent.
 3. A composition according to claim 2, wherein said bufferingagent is selected from the group consisting of phosphate, acetate,borate, citrate, phthalate, tetraborate, glycine salt, andtris(hydroxymethyl)aminomethane salt.